[sdiy] measuring pulse width with ns accuracy...

[Bert Schiettecatte]

hi magnus,

>Which frequencies?

does that really matter? let's say 25 Mhz or so.

>In principal this is pretty darn close to what interpolating reciprocal
counters
>do, like mine for instance.

that sounds like total chinese for me :-)

>The basic reciprocal counter acts on start and stop signals from the analog
>front-end. The time between the start and stop events is measured by
counting
>the number of reference clock cycles from the start event to the stop
event.

but to measurea 3ns interval you need a clock running at 300 Mhz or so?
counters you can buy off the shelf only go up to 170 Mhz or so.

>Gate-time prolongs this period so that averaging occurs, but the reciprocal
>counter checks out how many cycles have occured, so for frequency/period
>measures proper compensation can be handled.

Hmm.. I don't really understand this. Can you explain a bit more?

>Now, to get high time precission with a straight counter setup you would go
>into insane frequencies eventually. But there are tricks around it, many of
them
>infact. The winning contender in contemporary counters is analog
interpolators.
>They will measure the trime from the start/stop event (as received as a
signal
>edge) to the next reference-frequency edge. This is done by charging a
capacitor
>with a reference current. Then, the capacitor is discharged with another
(much
>lower) current. The time for the capacitor to discharge is measured by a
>separate counter as a number of clock cycles. By this technique will the
>pulse-differance be prolonged and the ratio is the ratio between the load
and
>discharge currents.

I'll have to do some thinking about this to fully understand what's going on
but I can see your point.

>Still more methods is at hand, all with their ups and downs. Consult
frequency
>control literature to learn more... or ask me ;O)

well, it's 1:30am here now so I think i'll get some sleep ;-) i'll read your
email
again tomorrow and try to understand it better. ah well.. maybe I should
have studied
analogue electronics instead of computer science...

thanks a lot for your help and suggestions. i'll try to prototype some
circuits and
let you know how it goes.

bert